Currents

Perhaps the most obvious type of current, surface currents are responsible for the major surface circulation patterns in the world's oceans. They are the result of the friction caused by the movements of the atmosphere over water; they owe their existence to the winds that form as a result of the warming of air masses at the sea surface near the equator and in temperate areas. When wind blows across the water's surface, it sets the water in motion. If the wind is constant and strong enough, the currents may persist and become permanent components of the ocean's circulation pattern; if not, they may be merely temporary. Surface currents can extend to depths of about 656 ft (200 m). They circle the ocean basins on both sides of the Equator in elliptical rotations.

There are several forces that affect and sustain surface currents, including the location of land masses, wind patterns, and the Coriolis effect. Located on either side of the major oceans (including the Atlantic, Indian, and Pacific), land masses affect currents because they act as barriers to their natural paths. Without land masses, there would be a uniform ocean movement from west to east at intermediate latitudes and from east to west near the equator and at the poles. The Antarctic Circumpolar Current can illustrate the west to east movement. Because no land barriers obstruct the prevailing current traveling between the southern tips of South America and Africa and the northern coast of Antarctica, the Antarctic Circumpolar Current consistently circles the globe in a west to east direction. Interestingly, this current is the world's greatest, flowing at one point at a rate of 9.5 billion cubic feet per second.

In addition to the presence of land barriers, two other factors work together to affect the surface currents—wind patterns and the Coriolis effect. The basic wind patterns that drive the currents in both hemispheres are the trade winds and the westerly winds. The Coriolis effect is a force that displaces particles, such as water, traveling on a rotating sphere, such as Earth. Thus, currents develop as water is deflected by the turning of Earth. At the equator, the effect is nonexistent, but at greater latitudes the Coriolis effect has a stronger influence. As the trades and the westerlies combine with the Coriolis effect, elliptical circulating currents, called gyres, are formed. There are two large subtropical gyres dominating each side of the equator. In the Northern Hemisphere, the gyre rotates in a clockwise direction; in the Southern Hemisphere, it rotates counterclockwise. At the lower latitudes of each hemisphere, there are smaller, tropical gyres which move in the opposite direction of the subtropical gyres.

A good illustration of a surface current is the Gulf Stream, also called the Gulf Current. This current is moved by the trade winds in the Atlantic Ocean near the equator flowing in a northwesterly direction. Moving along the coasts of South and North America, the Gulf Stream circles the entire Atlantic Ocean north of the equator. Currents similar to this exist in the Pacific Ocean and in the Atlantic south of the equator.

One of the major consequences of surface currents is their ability to help moderate the earth's temperatures. As surface currents move, they absorb heat in the tropical regions and release it in colder environments. This process is referred to as a net poleward energy transfer because it moves the solar radiation from the equator to the Poles. As a result, places like Alaska and Great Britain are warmer than they otherwise would be.